Transmission line with impedancematching terminations



1956 c. N. NEBEL 2,768,355

TRANSMISSION LINE WITH IMPEDANCE-MATCHING TERMINATION? Filed May 31,1952 //v l EN TOR Y C. N NE BE L A r TOEWE? United States Patent O lphone Laboratories, Incorporated, New Stork, Y., a corporation of NewYork Application May31; 1952", Serial Nb. 291,001 3' Claims- (C113538-27) This invention relates to wave transmission circuits and moreparticularly to impedance matching terminatio'ns for a transmissionline.

An objecto'f the invention is to avoid endreflections in a transmissioncircuit comprising a transmission-line operating between a source and aload, oneof which is larger and the other smaller than thecharacteristic impedance of the line.

More specifically, the object is tointroduce into such a circuitterminal impedances which will provide matching terminations for theline without causing transmission distortion due tothe added impedances:

In a wave transmission circuit comprising a transmission lineinterconnecting a wave source and a load neither of which matches thecharacteristic impedance Z of the line, undesired energy reflectionswill occur at the ends of the line. The impedance of either the sourceor the load may be either larger or smaller than Z0, which is assumed tohave a reactive component. The present invention is applicable to such acircuit when one of these impedances, preferably a pure resistance, hasa value Zs which is smaller than Z0 and the other has a value Zn whichis high compared to Z0. An impedance Z1 approximately equal to Z0Zs isconnected in series between the smaller impedance Zs and the line, andan impedance Z2 approximately equal to Z0 is connected in shunt with thelarger impedance ZH. In this Way, the line is terminated at each end inan impedance substantially equal to its characteristic impedance, thusgreatly reducing or eliminating end reflections. Furthermore, nodistortion of the over-all transmission loss characteristic isintroduced by the addition of these terminal impedances, because thecurrent through the series impedance Z1 at one end of the line producesa voltage loss characteristic which is entirely nullified by the voltageincrease characteristic due to the current through the shunt impedanceZ2 at the other end.

The nature of the invention and its various objects, features, andadvantages will appear more fully in the following detailed descriptionof typical embodiments illustrated in the accompanying drawing, of whichFig. 1 shows schematically a wave transmission circuit provided withimpedance-matching terminations in accordance with the invention;

Fig. 2 is a schematic circuit showing a second embodiment of theinvention; and

Fig. 3 shows schematically one network configuration suitable for theimpedances Z1 and Z2 of Figs. 1 and 2.

Taking up the figures in greater detail, Fig. 1 shows a wavetransmission circuit comprising a transmission line 1 of characteristicimpedance Z0 having a pair of input terminals 2-3 and a pair of outputterminals 4-5. The line 1 may, for example, be a coaxial or other typeof cable circuit the characteristic impedance of which has a reactivecomponent. A source 7 of wave energy of impedance Zs smaller than Z0 isconnected through a series impedance Z1 to the input terminals PatentedOct. zarase ice 2 3--of the line 1. The i' pedance Z1 has approximatelythe value Z1=Z0 -Zs- (1) An amplifier A 1. having an inputimpedance' Znlarge compared to Z0 is connectedxto the output terminals 45. Animpedance Z2, approximately equal to Z0, is shunted across the inputofthe amplifier A1 between the terminals 4-5 ofthe-line, and the outputof the ampli fier isconnected to a load imp'edanceZL.

The transmission line 1 is thus terminated at each end in an impedancesubstantially equal to its characteristic impedance Z0. The terminalimpedanceZT at thesend ing end is The impedance'm'atcli' at thisendwill,of course, depend upon how closely" Equation 1 is satisfied. Theterminal impedance: at the output end of the line is also equal" to Z0if the impedance Zn is high enough that its shunting effect ma'yjbeneglected. If this is not the'case, the-impedance'zz maybe made of sucha value thattlie parallel combination of Z2 and Zn. will be. equal toZ0. The reflection of energy at the ends of the line 1"d'ue to im- 7'pedaneemismatch ir thus greatly" reduced'or entirely eliminated. Theaddition of the impedances' Z1 and Z2 will cause no distortion of theover-all loss characteristic of the transmission circuit. The voltageloss characteristic due to the current through the series impedance Z1is exactly compensated by the voltage increase characteristic caused bythe current through the shunt impedance Z2.

Fig. 2 shows another embodiment of the invention in a transmissioncircuit which is similar to the one shown in Fig. 1 except that the highimpedance is here associated with the sending end of the line 1 and thelow impedance with the receiving end. In Fig. 2, signal waves from asource 8 are amplified in the amplifier A2 and impressed upon the inputterminals 2-3 of the line 1, to the receiving end of which is connecteda load impedance Zn. The amplifier A2 has an output impedance Zn whichis large compared with the characteristic impedance Z0 of the line 1.The load impedance Zs', which may represent receiving equipment or theinput impedance of an amplifier, is smaller than Z0. An

impedance Z2 approximately equal to Z0 is connected In this circuit,also, the voltage changes caused by the impedances Z1 and Z2 are equalbut opposite in sign throughout the frequency range of interest andtherefore have no net effect on the transmission loss characteristic ofthe circuit.

Fig. 3 shows a suitable network configuration for the impedance Z2 ofFig. 1. The impedance comprises three resistances R1, R2, and R3connected in series between the terminals 9 and 10. The resistance R1 isshunted by a capacitance C1 and the resistance R2 by a capacitance C2.The values of the component resistances and capacitances may be sochosen as to make the impedance be tween the terminals 9 and 10 simulatequite closely the characteristic impedance Z0 of the line 1 over theuseful The type of network shown in Fig. 3 may also be used for theimpedances Z1 and Z2 in Fig. 2.

Although the transmission circuits in Figs. 1 and 2 have been shown asunbalanced structures, it is to be understood, of course, that they maybe balanced. For example, if the circuit of Fig. 1 is balanced, theimpedance Z1 is replaced by two series impedances each equal to Z1/2,one of which is connected between the source 7 and the input terminal 2and the other between the source and the other input terminal 3.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

e 1. A wave transmission circuit comprising a transmission line ofcharacteristic impedance Z having a reactive component, a first terminalimpedance including a a wave source connected to one end of said line, asecond terminal impedance connected to the other end of said line, oneof said terminal impedances being a pure resistance of value Rs smallerthan Z0 and the other of said terminal impedances having a large valuecompared to Z0, an impedance equal over the frequency range of interestto ZoRs connected in series between said impedance of value Rs and saidline, and a twoterminal impedance equal over said range to Z0 connectedin shunt with said other terminal impedance.

2. A circuit in accordance with claim 1 in which said first terminalimpedance has the value Rs.

3. A circuit in accordance with claim 1 in which said second terminalimpedance has the value Rs.

References Cited in the file of this patent UNITED STATES PATENTS1,713,603 Hoyt May 21, 1929 1,815,254 Hoyt July 21, 1931 1,815,256 HoytJuly 21, 1931 2,158,978 Bowman-Manifold et a1. May 16, 1939. 2,163,750DHeedene June 27, 1939 2,223,736 Mertz Dec. 3, 1940 2,252,613 BingleyAug. 12, 1941 2,628,312 Peterson et a1 Feb. 10, 1953 FOREIGN PATENTS626,763 France May 21, 1927 454,463 Germany H--. Jan. 13, 1928

